1. Field of the Invention
The present invention is related to the field of amplifiers for magnetoresistance sensors.
2. Background Art
Preamplification circuitry is used to bias magnetoresistance (MR) sensors and to detect output signals generated by MR sensors. MR sensors are used as transducers for reading magnetically recorded data from disks having high-magnetization film media. MR sensors differ from inductive sensors in that they sense flux rather than change in flux per unit time. Since high density disk applications require closely spaced tracks of data, they exhibit lower signal flux than do lower density media. Despite the lower signal flux, MR sensors typically generate larger read signals than inductive heads in high density applications. Therefore, MR sensors provide improved read performance over inductive heads in high density recording applications.
An MR read sensor consists of a narrow stripe of material such as Ni-Fe which is mounted perpendicularly to disk media. The MR sensor has dimensions of height h and width w where the resistance of the sensor is inversely proportional to height h. The magnetoresistive effect causes the resistance of the sensor to vary according to magnetic flux from the media incident upon it.
Amplification circuitry provides a sense current to bias an MR sensor and amplifies read signals produced by the sensor. The sense current for an MR sensor can be supplied by means of constant voltage or constant current. Similarly, the electronic sensing of a resistance signal can take two essentially different forms: detecting the voltage across the sensor by means of a high input impedance voltage amplifier or detecting the current through the sensor by means of a current amplifier with a virtually shorted input.
Preamplification circuitry provides a sense current which is applied to an MR sensor to bias its response to produce an optimum rate of change in resistance with respect to a magnetic field. Therefore, data stored as magnetized locations on a disk causes the resistance of an MR sensor to vary as the data passes under the MR read mechanism. Preamplification circuitry detects changes in the voltage across the MR sensor due to changes in its resistance and amplifies this read signal generated by the sensor. Thus, an MR sensor properly biased by amplification circuitry provides a linear response to the instantaneous magnetic field of disk media passing by the sensor.
Because the resistance of MR sensors is inversely proportional to height h, it is desirable to bias the sensor so that it is insensitive to variations in the height of stripes due to manufacturing processes or due to wear. Thus, problems associated with differences in the height of MR sensors may be reduced. In particular, it is desirable to generate a sensor signal having a voltage which is proportional to the change in sensor resistance normalized by its resistance, .DELTA.R.sub.h /R.sub.h, where .DELTA.R.sub.h is the change in MR sensor resistance, R.sub.h, from its steady-state resistance when a magnetic field is incident upon the MR sensor. It is noted that the steady state value of R.sub.h refers to the resistance value of R.sub.h while biased, but not being subjected to a magnetic field.
A diagram illustrating a prior art pre-amplifier is illustrated in FIG. 1. The circuit includes NPN transistors 107 and 108, amplifier 106, MR sensor 110, capacitor 109 and current source 111. Voltage rail 101 at a voltage of +V.sub.S is coupled to the first terminal of resistor 102 and the first terminal of resistor 103. Both resistors 102 and 103 have a value of R.sub.C. The second terminal of resistor 102 is coupled to output terminal 104, the inverting input of transconductance amplifier 106 and the collector of NPN transistor 107. The second terminal of resistor 103 is coupled to output terminal 105, the non-inverting input of transconductance amplifier 106 and the collector of NPN transistor 108. The output of amplifier 106 is coupled to the first terminal of capacitor 109 and the base of transistor 108. The second terminal of capacitor 109 is coupled to ground.
The base of transistor 107 is coupled to ground. The emitter of transistor 107 is coupled to the first terminal of current source 111. The emitter of transistor 108 is coupled to the first terminal of MR sensor 110. The resistance of MR sensor 110 has a value of R.sub.h. The second terminal of MR sensor 110 is coupled to the first terminal of current source 111. A current of 2I.sub.B flows from the first terminal to the second terminal of current source 111. The second terminal of current source 111 is coupled to voltage rail 112. Voltage rail 112 has a voltage of -V.sub.S. The differential output signal seen at output terminals 104 and 105 is V.sub.OUT.
FIG. 1 illustrates a prior art circuit that biases MR sensor 110 having resistance R.sub.h and amplifies the signal generated by the MR sensor 110. Current source 111 sinks emitter currents from transistor 107 and 108 equal to a constant value 2I.sub.B. MR sensor 110 causes a current imbalance in the two current paths through the differential pair resulting in a voltage across output terminals 105 and 104. The output signal V.sub.out produced by the differential pair is fed back through amplifier 106 and capacitor 109 to the base of transistor 108 to correct for DC offsets due to the steady-state resistance value of MR sensor 110 and variations in transistors 107 and 108. This feedback produces balanced current flow through each path of the differential amplifier. The low frequency response of this configuration is modified by adjusting the gain of amplifier 106 and capacitor 109 of the feedback loop in order to set the dominant pole appropriately.
A disadvantage of the prior art circuit is the considerable recovery time required to stabilize such a circuit as shown in FIG. 1 when switching from one sensor to another. This is accomplished by deactivating one sensor and associated input stage and activating another. The prior art describes a circuit for moving the dominant low frequency pole during switchover. However, the recovery time is still considerable due to the fact that the feedback loop control signal is applied to the input stage differentially. Thus, the transient at the output of the differential stage must decay sufficiently to resolve low level read signals, and this process requires substantial time. Another disadvantage is that the prior art amplifier requires additional, non-linear circuitry, which increases the complexity of the amplifier.